But, with more tabs opened the memory usage increases, but when closing these tabs, memory doesnt drop down? how come?

The GRE that K-Meleon is using reserves idle memory for immediate reaction as long as system does not need it back. The memory is not lost - just reserved.

You can see that if You minimize the browser. It releases memory because that action makes it assume that spare memory to react is no more needed for a while.

There is however one K-Meleon extension called AdblockPlus that has been "bleeding" loosing memory. That memory is not released any more. I do not know whether this bug is fixed already. I do not use the extension.